Centile calculator for mean uterine artery pulsatility index at 11-13 weeks
The data were derived from the study of 21,673 singleton pregnancies with a live fetus at 11+0 - 13+6 weeks and no major defects or chromosomal abnormalities. The Doppler studies were carried out transabdominally.
Background
Uterine artery PI provides a measure of uteroplacental perfusion and high PI implies impaired placentation with consequent increased risk of developing preeclampsia, fetal growth restriction, abruption and stillbirth. The uterine artery PI is considered to be increased if it is above the 90th centile.
In normal pregnancy the uterine artery PI decreases with fetal crown-rump length and maternal weight, and it is increased in women of African racial origin. In assessing whether a measurement is normal or not these maternal characteristics should be taken into consideration.
Measurement of uterine artery PI
A sagittal section of the uterus should be obtained and the cervical canal and internal cervical os should be identified. The transducer should be gently tilted from side to side and color flow mapping should be used to identify each uterine artery along the side of the cervix and uterus at the level of the internal os. Pulsed wave Doppler should be used with the sampling gate set at 2 mm to cover the whole vessel and care should be taken to ensure that the angle of insonation is less than 30ยบ. When three similar consecutive waveforms are obtained, the PI and peak systolic velocity (PSV) should be measured and the mean PI of the left and right arteries calculated. The uterine artery PSV should not be less than 60 cm/s. A lower measurement indicates that a wrong vessel has been examined.
It is strongly recommended that those undertaking assessment of risk receive the Fetal Medicine Foundation Certificate of competence in the 11-13 weeks scan and in Doppler ultrasound (see FMF Certificates of Competence)
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